Restoration of tooth structures (enamel, dentin, and cementum) damaged, for example, by dental caries typically uses a restorative filling material such as a filling composite resin or filling compomer or a crown restoration material such as a metal alloy, porcelain, or resin material. In general, however, restorative filling materials and crown restoration materials (both of these materials may collectively be referred to as “dental restorative material(s)” in the present description) themselves have no bonding ability to tooth structures. This is why bonding between tooth structures and dental restorative materials conventionally employs various adhesive systems involving the use of adhesives. An example of conventionally-employed adhesive systems is an adhesive system of the so-called etching-type, the use of which consists of subjecting the surface of a tooth structure to an etching treatment with an acid etching agent such as an aqueous phosphoric acid solution, then applying a bonding material which is an adhesive to the tooth structure, and bonding a dental restorative material to the tooth structure.
In recent years, adhesive systems of the so-called self-etching type, which involve no use of any acid etching agent, have been widely employed. Such adhesive systems include: a two-step adhesive system the use of which consists of applying a self-etching primer containing an acidic monomer, a hydrophilic monomer, and water to the surface of a tooth structure and then applying a bonding material containing a crosslinkable monomer and a polymerization initiator to the tooth structure without washing with water; and a one-step adhesive system involving the use of a one-part dental adhesive (one-part bonding material) having functions of both a self-etching primer and a bonding material.
The use of the two-step adhesive system typically consists of applying and drying a primer on a part to be repaired, then applying a bonding material to the part, and causing photocuring. Thus, the two-step adhesive system involves the use of a photopolymerization initiator. Patent Literature 1 discloses that using an acylphosphine oxide compound as a photopolymerization initiator in a bonding material of a two-step adhesive system allows quick, strong hardening and leads to high strength and durability of bond to a tooth structure even when the primer contains no photopolymerization initiator. Also, Patent Literature 2 discloses that using a combination of an acylphosphine oxide compound with an α-diketone compound as a photopolymerization initiator in a bonding material of a two-step adhesive system allows quick, strong hardening and leads to high strength and durability of bond to a tooth structure even when the primer contains no photopolymerization initiator. Examples of Patent Literature 2 present a result indicating that the addition of an α-diketone compound provides an increase in bond durability. However, the use of a photopolymerization initiator that induces quick hardening may disadvantageously cause shortening of allowable operation time since the bonding material containing such a photopolymerization initiator may have too high light sensitivity and hence low stability to ambient light such as light from a fluorescent lamp or digital lamp in a dental office. Thus, a bonding material containing a photopolymerization initiator is required to have a good balance between two conflicting properties, ambient light stability and photocurability. In view of ambient light stability and photocurability, paragraph 0028 of Patent Literature 2 discloses using 0.01 to 0.5 parts by weight of the α-diketone compound per part by weight of the acylphosphine oxide compound.
Halogen irradiation devices have been conventionally used as dental irradiation devices. In recent years, however, LED irradiation devices, which are characterized by long lamp life and high light use efficiency, have become increasingly used as an alternative to halogen irradiation devices. A beam of light emitted from an LED irradiation device has a narrower range of wavelengths and a different emission spectrum than a beam of light emitted from a halogen irradiation device, although these beams of light have similar peak wavelengths. A dental photocurable material thus shows different curing behaviors depending on which of an LED irradiation device and a halogen irradiation device is used to cure the material. Over the last few years, the output power of LED irradiation devices has been increasing, and high-power LED irradiation devices that allow shortening of the curing time of photopolymerizable resins have become increasingly widespread.
The two-step adhesive systems disclosed in Patent Literature 1 and 2 mentioned above show good photocurability when subjected to irradiation with a halogen irradiation device that used to be common. The adhesive systems, however, have been found to have a disadvantage for photocuring with an LED irradiation device in that imparting a satisfactory level of ambient light stability leads to a reduction in the rate of photocurability. The adhesive systems have also been found to pose the problem of low level and large variability of bond strengths to enamel and dentin which are obtained by short-time irradiation with a high-power LED irradiation device.
Patent Literature 3 proposes a composition that exhibits high photocurability even when irradiated with an LED irradiation device, the composition including a photopolymerization initiator consisting only of a combination of bisacylphosphine oxide and α-diketone. Patent Literature 3 also discloses that the composition is available as a bonding material.
A study by the present inventors, however, has revealed that the composition of Patent Literature 3 has room for improvement. To be specific, the composition has low ambient light stability, although it shows a high rate of photocuring when irradiated with an LED irradiation device. In particular, the composition has markedly low ambient light stability when used as a bonding material in a two-step adhesive system.
Patent Literature 4 proposes a photopolymerizable composition having high stability to ambient light, the composition containing α-diketone, acylphosphine oxide, an aromatic tertiary amine, and a benzotriazole compound. Patent Literature 4 discloses that a preferred weight ratio between the α-diketone and the acylphosphine oxide (α-diketone:acylphosphine oxide) is 1:2.5 to 10.
The above composition of Patent Literature 4 is intended for use as a dental restorative material rather than use in a dental adhesive system, and is effectively available as a hydrophobic composition such as a dental composite resin. However, a composition used in a dental adhesive system typically contains a hydrophilic component essential in terms of penetration into collagen of dentin and compatibility with tooth structures. A composition containing both a hydrophilic component and a benzotriazole compound generally has low state stability and suffers deposition of the benzotriazole compound after storage for a short period of time. The composition of Patent Literature 4 is thus difficult to practically use in a dental adhesive system, which fact has been previously known.